I too get the expected object with a hole in it.

Perhaps you have View/Thrown Together set?  That makes it look like the
objects are combined instead of differenced.

A few comments on the program...

On 4/15/2021 8:52 PM, sorin vatasoiu via Discuss wrote:

360 is a lot for $fn.  In more complex models, that's likely to cost you
a lot of time.

With $fn=360, your model has 412 facets and looks like this:

With $fa=1 and $fs=1, your model has 115 facets and looks like this:

You can see the difference on the screen.  My bet is that you can't see
the difference in plastic.

Roughly, $fa controls the behavior of large circles and $fs controls the
behavior of small circles.  The main body of the object ends up the same
in either variation, with 1° segments.  The difference is in the hole,
which has 360 segments in one variation and about 60 in the other. 
Those 60 segments are each 1mm long.

You've asked for a cone with a bottom radius of 10 and a top diameter of
20.  It's unusual to specify a radius for one and a diameter for the
other, but more importantly those are the same size so you could have
just used r=10 or d=20.

I couldn't find references to $fn, $fa, and $fs on the OpenSCAD cheat
sheet: would it make sense to include them there?

On 4/16/2021 12:48 AM, Jordan Brown wrote:

JordanBrown wrote

I tend to use radius or diameter interchangeably, depending on how I think
of the way the part fits other parts, or the use of the part of the part
itself. Yes, I might even use radius and diameter for opposite ends of a
cylinder for the same reason. But yes, if they are the same size, it's
better for readability to make them the same units.

--
Sent from: http://forum.openscad.org/

My bad: they are already present

On 4/16/2021 3:06 PM, jon wrote: